The present invention relates generally to various semiconductor-processing-compatible infrared (IR) sensor structures and fabrication methods, and more particularly to improved IR radiation sensing structures and processes which reduce size and cost of IR sensors and which provide smaller, more economical, more sensitive IR radiation intensity measurements. More particularly, the invention also relates to improved IR radiation sensing structures and processes which also avoid the costs and difficulties associated with discrete lenses as used in the closest prior art to collimate or shift the angle of incoming IR radiation, by integrating diffractive optical elements, such as Fresnel lenses or diffraction gratings, into an integrated circuit chip including an IR sensor.
The closest prior art is believed to include the article “Investigation Of Thermopile Using CMOS Compatible Process and Front-Side Si Bulk Etching” by Chen-Hsun-Du and Chengkuo Lee, Proceedings of SPIE Vol. 4176 (2000), pp. 168-178, incorporated herein by reference. Infrared thermopile sensor physics and measurement of IR radiation using thermopiles are described in detail in this reference. “Prior Art” FIGS. 1A and 1B herein show the CMOS-processing-compatible IR sensor integrated circuit chip in FIGS. 1 and 2, respectively, of the foregoing article. The closest prior art also is believed to include the Melexis MLX90614 “infrared thermometer” in a TO-39 package, generally as shown in “Prior Art FIG. 2. Information regarding fabrication of several kinds of Fresnel lenses, including Fresnel Zone Plate binary lenses, Phase Zone Plate binary lenses, and Phase Fresnel Lenses, in/on silicon is disclosed in the technical article “Development of a Deep Silicon Phase Fresnel Lens Using Gray-Scale Lithography and Deep Reactive Ion Etching” by Brian Morgan et al., Journal of Micro-Electromechanical Systems, Vol. 13, No. 1, February, 2004.
Referring to Prior Art FIG. 1A herein, the IR sensor chip includes a silicon substrate 2 having a CMOS-processing-compatible dielectric (SiO2) stack 3 thereon including a number of distinct sub-layers. A N-type polysilicon (polycrystalline silicon) trace 11 and an aluminum trace M1 in dielectric stack 3 form a first thermocouple junction where one end of the polysilicon trace and one end of the aluminum trace are connected. (A “thermocouple group”, as that term is used herein, is formed on a group of series-connected thermocouples. A “thermopile”, as the term is used herein, is formed of a number of thermocouple groups connected in series.) Additional oxide layers and additional metal traces also may be included in dielectric stack 3. An oxide passivation layer 12A is formed on top of dielectric stack 3, and a nitride passivation layer 12B is formed on oxide passivation layer 12A. A number of silicon etchant openings 24 extend through nitride passivation layer 12 and dielectric stack 3 to the top surface of silicon substrate 2 and are used to etch a cavity 4 in silicon substrate 2 underneath the portion of dielectric stack 3 in which the thermopile is formed, to thermally isolate it from silicon substrate 2.
Prior Art FIG. 1A is taken along section line 1A-1A of Prior Art FIG. 1B, which is essentially similar to FIG. 2 of the above mentioned Du and Lee reference. Cavity 4 is etched underneath SiO2 stack 3 by means of silicon etchant introduced through the various etchant openings 24, which are relatively large and irregular. FIG. 1B shows various metal-polysilicon strips MP1 each of which includes an aluminum strip M1 and a polysilicon strip 11 which makes electrical contact to the aluminum strip M1 as shown in FIG. 1B. The metal strips M1 run parallel to the polysilicon strips 11 and, except for the electrical contact between them as shown in FIG. 1A, are separated from polysilicon strips 11 by a sublayer of SiO2 stack 3. Although not shown in FIG. 1A, the dielectric material directly above metal strips M1 actually has corresponding steps which are indicated by reference numerals MP2 in FIG. 1B. The relatively large etchant openings 24 and their various angular shapes cause the “floating” membrane consisting of the various metal-polysilicon strips MP1 and the central section 3A of SiO2 stack 3 supported by metal-polysilicon strips MP1 to be very fragile. Such fragility ordinarily results in an unacceptably large number of device failures during subsequent wafer fabrication, subsequent packaging, and ultimate utilization of the IR sensor of FIGS. 1A and 1B.
A second thermocouple group (not shown) essentially similar to the one shown in FIG. 1A is also formed in dielectric stack 3 directly over a silicon substrate 2 and is not thermally isolated from silicon substrate 2 and therefore is at the same temperature as silicon substrate 2. The first and second thermocouple groups are connected in series and form a single “thermopile”. The various silicon etchant openings 24 are formed in regions in which there are no polysilicon or aluminum traces, as shown in the dark areas in FIG. 2 of the Du and Lee article.
Incoming IR radiation indicated by arrows 5 in Prior Art FIG. 1A impinges on the “front side” or “active surface” of the IR sensor chip. (The “back side” of the chip is the bottom surface of silicon substrate 2 as it appears in Prior Art FIG. 1A.) The incoming IR radiation 5 causes the temperature of the thermocouple junction supported on the “floating” portion of dielectric membrane 3 located directly above cavity 4 to be greater than the temperature of the second thermocouple junction (not shown) in dielectric membrane 3 which is not insulated by cavity 4.
Unfortunately, the floating membrane supporting one of the group of thermocouple junctions over the cavity as shown in FIG. 1 of Du and Lee (Prior Art FIG. 1A herein) is very fragile, because of the irregular sizes and spacings of the etchant openings and partly by the thinness of the dielectric layer 3.
The IR radiation sensor in Prior Art FIG. 1A measures the temperature difference T1-T2 and produces an output voltage proportional to that temperature difference. The aluminum trace and N-type polycrystalline silicon trace of which the first and second thermocouple junctions are formed both are available in a typical standard CMOS wafer fabrication process.
FIG. 3 of the Du and Lee article indicates that the IR sensor is mounted inside a metal package having a window through which ambient IR radiation passes to reach the thermopile in the packaged IR sensor chip. The IR sensor chip described in the Du and Lee article is not believed to have ever been commercially available.
Above mentioned Prior Art FIG. 2 shows a commercially available MLX90614 IR radiation sensor marketed by Melexis Microelectronic Integrated Systems. This device is packaged in a metal TO-39 package 9 having a planar silicon window 10 through which impinging IR radiation can pass in order to reach the packaged IR sensor.
The above described prior art IR sensors require large, expensive packages. The foregoing prior art IR radiation sensors need to block visible light while transmitting IR radiation to the thermopiles in order to prevent false IR radiation intensity measurements due to ambient visible light. To accomplish this, the packages typically have a silicon (or other material transmitting infrared radiation but blocking visible light) window or a window with baffles. Furthermore, the “floating” portion of dielectric membrane 3 over cavity 4 in Prior Art FIG. 1A is quite fragile. In many of the prior art IR sensors, the silicon cavity is etched from the “back side” of the silicon wafer. This creates a large opening span that is difficult to protect.
The prior art also is believed to include use of a discrete Fresnel lens in conjunction with an infrared sensor of the kind typically used in motion sensing alarms. Such devices include a relatively large plastic Fresnel lens, and the infrared sensor typically is a pyroelectric sensor which is substantially different than a thermopile sensor. The discreet Fresnel lens is used to collimate ambient IR radiation from a relatively large, distant source and focus it on to a relatively small IR sensor to improve its signal to noise ratio. Somewhat similarly to the Melexis prior art shown in FIG. 2, the commercially available Omega OS36-10-K infrared thermocouple is packaged in a cylindrical housing along with a conventional classical curved lens to collimate the incoming infrared radiation in order to increase the thermocouple sensitivity.
Alignment of a small thermopile sensor of the kind shown in the above mentioned Du and Lee article or included in the Melexis IR thermometer with a discrete collimating lens is difficult and expensive because of the relatively small size of the thermopile membrane. The thermopile might have a dimension of only approximately 200 microns across, so misalignment of the discrete collimating lens by more than approximately 50 microns may result in substantial errors in focusing the incoming IR radiation onto the thermopile. Furthermore, if the collimating lens is located a few centimeters from the thermopile, tilt of more than a few degrees of the plane of the collimating lens with respect to the plane of the thermopile will cause the focal point to be substantially misaligned with the thermopile. The degree of precision alignment typically needed is achievable but quite costly using conventional alignment methods.
It would be highly desirable to provide smaller, more economical, more sensitive, and more robust IR sensors than are known in the prior art for various applications such as non-contact measurement of temperature and remote measurement of gas concentrations.
Thus, there is an unmet need for an IR radiation sensor which is substantially smaller, more sensitive, and less expensive than the IR radiation sensors of the prior art.
There also is an unmet need for an IR radiation sensor which avoids the costly problems associated with the packaging of the prior art sensors and the alignment of discrete diffractive optical elements with thermopiles, thermocouple groups, or even individual thermocouples of the sensors.
There also is an unmet need for a low cost IR radiation sensor having a response which is sensitive to the wavelength of incoming IR radiation.
There also is an unmet need for a low-cost IR radiation sensor which is substantially smaller and more sensitive than the IR radiation sensors of the prior art, wherein the IR-radiation-sensitive elements are composed of thermocouples.
There also is an unmet need for a low-cost IR radiation sensor which is substantially smaller and more sensitive than the IR radiation sensors of the prior art, wherein the IR-radiation-sensitive elements are composed of temperature-sensitive resistive elements.
There also is an unmet need for a more accurate IR radiation sensor than has been found in the prior art.
There also is an unmet need for an IR radiation sensor which provides more sensitive, more accurate measurement of IR radiation than the IR radiation sensors of the prior art.
There also is an unmet need for a CMOS-processing-compatible IR radiation sensor chip which does not need to be packaged in a relatively large, expensive package having a discrete window.
There also is an unmet need for a CMOS-processing-compatible IR radiation sensor chip which is substantially more robust than those of the prior art.
There also is an unmet need for an improved method of fabricating an infrared radiation sensor.
There also is an unmet need for an improved method of fabricating a CMOS-processing-compatible IR sensor device which does not require bonding the CMOS-processing-compatible IR sensor chip in a relatively large, expensive package having an infrared window therein.